The thermal conductivity detector (TCD) is well known in the state of the art. A TCD is an environmental sensor device widely used for the measurement of the amount of gas in the environment. The operation is based on the fact that each gas has an inherent thermal conductivity and a filament (thermal resistor) changes its temperature as a function of the amount of gas that surrounds it. The most appropriate sensing element shape is that of a thin finger suspended, for which the temperature of the central part can locally reach even values of several hundred degrees. The feature that the finger is totally suspended allows for enhancing the amount of heat exchange with the gas in which it is immersed. The warming effect of the suspended finger is induced through an electrical stress of the sensor, for example by means of the flow of current through the finger. The sensor is able to better discriminate the gases whose conductivity is much different than normal air (roughly N2 (79%), O2 (19%), CO2 (0.04%), plus other gases with negligible quantities: for example the CO is a few ppm).
When a current flows through the finger, the value of the resistance of the finger changes. The measurement of the resistance value allows for measuring the conductivity of the gas mixture which depends on the molar fraction of the gas of interest.
However, it is difficult in principle to discriminate which is the gas mainly responsible for the conductivity variation of the mixture of gas. For example, CO2 has a lower thermal conductivity than dry air, therefore if its percentage increases inside the mixture, this will raise the temperature of the sensor with a consequent increase of the value of the measured resistance.
The TCD sensor operates in accordance with the thermodynamic equilibrium among heat generated by the current flow, heat exchange with the material of which the sensor is made (e.g., polysilicon crystalline), and heat exchange with the gas mixture surrounding it. The ambient temperature determines the equilibrium value of the sensor in standard dry air. To take into account and compensate for the variation of ambient temperature, a Wheatstone bridge could be used as the sensor structure. The reference branches of the bridge are of the same nature and positioned in the vicinity of the sensor so as to be sensitive to the same way to changes in ambient temperature, with the difference that will not be exposed to the mixture of gas as the sensor.
The Relative Humidity (RH) is the amount of water vapor (gas) present in the environment compared to a saturated environment in the same conditions of pressure and temperature. The thermal conductivity of water vapor is much larger than the dry air therefore an increase in relative humidity produces a lowering of the temperature of the sensor with a consequent reduction of the value of the measured resistance. The contribution of the RH value of the measured resistance could be 1/10 compared to the change of resistance in the presence of CO2, therefore, this is a parameter to measure and correct. Typically the correction is made by means of a dedicated sensor for the measurement of the RH.
In view of implementation of space saving and low power consumption, a demand exists to further reduce the size of gas detectors for measuring the concentration of gas. In recent years, gas detection elements with greatly reduced sizes have been developed by the use of MEMS (Micro-Electro-Mechanical System) technology (also called the micromachining technique). A gas detection element formed by use of MEMS technology is configured such that a plurality of thin films are formed in layers on a semiconductor substrate (e.g., a silicon substrate). Examples of such a gas detection element include a thermal-conductivity-type gas detection element. The thermal-conductivity-type gas detection element has a heat-generating resistor and utilizes the phenomenon that, when the heat-generating resistor is energized and generates heat, heat is conducted to the gas. The conduction of heat causes a change in temperature of the heat-generating resistor and thus a change in resistance of the heat-generating resistor. On the basis of the amount of the change, the gas is detected. In the thermal-conductivity-type gas detection element, the resistance of the heat-generating resistor varies with the type or concentration of the gas.